1. Field
The present disclosure relates generally to radio frequency (RF) power amplifiers, and in particular, to a system and method of performing adaptive digital predistortion (DPD) of complex modulated waveform based on metrics of a system, such as average power or root mean square (RMS) voltage and peak power or voltage from the output of an RF power amplifier.
2. Background
In the wireless communications field, there is a general need for devices capable of transmitting more data within a given bandwidth, and at the same time achieving a reasonable or optimal power efficiency to conserve battery power. For instance, wireless devices have been designed with different modulation schemes, such as quadrature amplitude modulation (QAM) having 16, 32, or 64 constellations, to increase the data throughput within a given bandwidth. Additionally, wireless devices have also been designed using power amplifiers that are operate close to their saturation region, such as class A/B, B, C, and other class amplifiers, to improve the power consumption efficiency.
Because of the relatively high spectral efficiency of the data transmission, such wireless devices often have tight requirements on the allowable spectral leakage. In some cases, these requirements present a problem for operating power amplifiers close to their saturation regions because the nonlinearity characteristic of the amplifier causes significant spectral re-growth and in-band distortion. One solution is to backoff the operation of the amplifier into its linear region so as to reduce or prevent this distortion. However, this results in reduced power efficiency for the device, which has adverse impact on the battery life and continued use of the device.
Another solution is to operate the power amplifier near its saturation or nonlinear region, and use a predistortion device at the input of the amplifier to distort the input signal so as to correct or reduce the distortion of the output signal caused by the nonlinearity of the amplifier. There are generally two approaches: an open loop approach and a closed loop approach. The open loop approach typically works well as long as the nonlinear characteristic of the amplifier is accurately modeled and does not significantly change over time with environmental conditions. The closed loop approach involves providing adaptation to the predistortion device so that it can model the nonlinear characteristic of the power amplifier in “real time,” and adjust the predistortion of the input signal in accordance with the present model of the amplifier. However, often these adaptation techniques are complicated and expensive, as discussed as follows.
FIG. 1 illustrates a block diagram of a typical closed loop transmitter system 100 that uses a demodulation technique to provide information about an output signal in order to apply predistortion of an input signal to compensate for distortion in the output signal caused by a power amplifier. In particular, the transmitter system 100 includes a digital predistortion (DPD) device 102, a digital-to-analog converter (DAC) 104, an automatic gain control (AGC) 106, an up converting mixer 108, and a power amplifier 110. The transmitter system 100 further includes a demodulation section including a power splitter 112, a pair of mixers 114 and 116, an oscillator 120, a 90° phase shifter 118, and a pair of filters 122 and 124.
The DPD device 102 predistorts an input baseband or intermediate frequency (IF) digital signal based on signals received from the demodulation section in order to achieve a target signal at the output of the power amplifier 110. The DAC 104 converts the predistorted digital signal from the DPD device 102 into an analog signal. The AGC 106 dynamically amplifies or attenuates the analog signal in order to achieve a target power level for the signal at the output of the power amplifier 110. The up converter mixer 108 uses a local oscillator (L.O.) to upconvert the baseband or IF analog signal into a radio frequency (RF) signal. The power amplifier 110 amplifies the RF signal to generate an output RF signal.
The demodulation section converts a sampled portion of the output RF signal into an I/Q IF or baseband signals for use by the DPD device 102 in predistorting the input digital signal to achieve a target RF output signal for the transmitter 100. The power splitter 112 splits the sampled output RF signal into two components for processing by the I- and Q-portions of the demodulation section. The mixer 114 uses the signal from the oscillator 120 to down convert the sampled output RF signal into an I-component IF or baseband signal. The filter 122 removes high order frequency components from the I-signal. Similarly, the mixer 116 uses the signal from the oscillator 120 shifted in phase by 90 degrees by the phase shifter 118 to down convert the sampled output RF signal into a Q-component IF or baseband signal. The filter 124 removes high order frequency components from the I-signal.
There are many drawbacks with the demodulation approach. For instance, the circuitry is very complex requiring a demodulation section to generate I- and Q-IF or baseband signals for use by the DPD device in predistorting the input digital signal to achieve a target output signal. The complexity is further underscored by the fact that the I- and Q-signals should be time aligned with the input signal for the system to operate properly. Further, the I- and Q-demodulation generally requires predistortion both in the amplitude domain and in the phase domain. Usually, a higher resolution DAC is required when predistorting of the input signal occurs in both the amplitude and frequency domains.